Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
BACKGRt)UND OF THE INVENTION
This invention relates generally to pollution
control systems and, more particularly, to a pollution
control system for the removal of sulfur oxides from flue
gases and the reduction of the sulfur oxides to sulfur.
It is known in the field of atmospheric pollution
control to use an adsorptive process for the desulfurization
of flue gases in which the sulur-containing material is
adsorbed in the porous system of an activated carbonaceous
material. In one such process, adsorption is carried out in
a gas-solid contacting device in which the flue gases are
contaoted with acti~ated char and sulfur dioxide in a
diluted form in the gas stream passing through the activated
char is adsorbed and oxidized to sulfuric acid by the oxygen
~` 15 and water vapor present in the gas stream. Qther acid gases,
such as nitrogen oxides, are similarly adsorbed, and
particulate matter entrained in the gas stream is filtered
by passage of the stream through the activated char.
In the prior art process developed by 3erg~au
Forshung and depicted in Figure l, the acid-laden or saturated
char is thermally regenerated in a desorption vessel, or the
like, by a process in which the sulfur-containing material is
chemically changed in form, resulting in the decomposition
of sulfuric acid to sulfur dioxide and water, whereby a
portion of the carbonaceous adsorbent is oxidized to carbon
dioxide. The by-product of the regeneration process is a gas
stream containing 20-30% by volume of sulfur dioxide, which
is directed to an off-gas treatment facility for further
pxocessing.
2-
The prior art system of Fig. 1 is composed
of three basic subsystems: adsorption, regeneration or
desorption, and off-gas treatment. In the adsorption
subsystem, an adsorber 10 recelves flue gases from the
S vapor generator after they have passed through a particulate
matter separator, or the like (not shown), and the flue
gases are contacted with adsorbent material in pellet
form loaded into the adsorber. The adsorbent material
used in the adsorber 10 is usually in the form of a pre-
oxidi~ed bituminous coal, or activated char.
In the regeneration subsystem, an inert, heatexchange medium, such as sand, is heated in a sand heater 12
to a predetermined, elevated temperature, and is supplied
to a regenerator 14, through which the heated sand and the
saturated char pellets pass in intimate contact. This
contact raises the temperature of l:he mixture to a pre-
determined level to cause the sulfuric acid in the porous
system of the activated char to be converted first to sulfuric
acid anhydrate (~2SO3) and then to SO2, and the nitrogen
comp~unds to N2. A highly~concentrated, SO2-rich off-gas
stream is produced, containing 20-30~ by volume, and is
usually directed to an exterior unit for further processing.
The sand/char mixture leaving the regenerator 14 goes
through a separator 16, which separates the regenerated
char from the sand. The separated sand is returned to the
sand heater 12, again heated to the proper elevated tempera-
ture, and recycled into the regenerator 14. The separated
char is directed to a char cooler 18, in which it is cooled
and recycled to the adsorber 10 for re-use.
In this type of prIor a~t syste~ the S02-rich
off-gas is usually treated further to produce elemental
sulfur, which is storable and which has certain commercial
applications. To this end the off-gas can be reacted with
crushed coal to produce the elemental sulfur. Fo~ example,
in the system disclosed in the above identified application,
the S02-rich off-gas is introduced into a reactor, shown
in general by the reference numeral 20 in Fig. 1, and is
initially reacted with crushed coal which is continuously
supplied to the vessel to yield gaseous elemental sulfur,
which i5 then passed to a condenser 22 and condensed into
liquid sulfur. The li~uid sulfur may be stored in appro-
priate containers, or may be cooled into solid form.
In the above-described system of Fig. 1, a certain
~uantity of the acti~ated char is consumed by the chemical
; reaction which occurs in the regenerator 14, and as a result
of the continuous recycling of the regenerated char, a
portion of this material becomes physically reduced to such
a size which renders it ineffective in the adsorption process.
Thus, a source of additional activated char is provided
to the adsorber 10 to replenish the char consumed in the
regenerator 14 and to make up for the quantity which is
; physically reduced. (In the reactor 20, the crushed coal
supplied thereto is consumed in the reaction process, yield-
ing coal ash as a by-product, which is not otherwise utilized
in the process.)
The major problem w~th the pr~or art process of
Fig. 1 resides ln the inherent difficulty in separating
the regenerated adsorbent from the sand for recycle to the
adsorber. Also, with increased emphas~s on encrgy and
.~
. 3 ~31~D.~O .~D
natural resource conservation, a need exists for replacing that
commercial proces~ with a process which is more energy
efficientJ specifically, one which achieves regeneration wi-th
like efficiency at lower temperature.
It is also well known in the art to regenerate
activated carbons by contact with a hot regenerating gas. For
example, U.S. 2,933,454 discusses regenera-tion with mixtures of
air and steam. ~owever, where it is desirable to treat the
desorbed gas for recovery, for example, o sulfur value,
regeneration of the adsorbent by contact with a hot regenerating
gas poses a problem of dilution of the desorbed volatiles to the
point where they cannot be economically treated for the
recovery of some value contained therein. One solution to this
problem is disclosed in U.S. 3,667,910 wherein it is taught that
the regenerating gas may be recirculated through the desorber to
elevate the concentration of sulfur dioxide to facilitate
recovery (Col. 4, lines 40 - 45). lrhe problem with this prior
art process is that to achieve adequately effective
regeneration, carbon monoxide gas and/or hydrogen gas must be
continuously generated and combined with the circulating
desorbent.
SUMMARY OF THE INVENTION
. _ _
The present invention comprehends a process for
desulfurizing a flue gas with a carbonaceous adsorbent and
reactivating the spent carbonaceous adsorbent. The process
-
i~, ~5~
f~ 9;2
comprises using a partially oxidized coal as the carbonaceous
adsorbent and continuously passing the spent adsorbent through a
regeneration zone as a non-static bed. The process further
comprises passing an initial charge o~ an inert gas into a
regenerating gas loop and through the regeneration zone in a
direction countercurrent to the flow of -the spent adsorbent for
contact with the spe~t adsorbent to desorb adsorbed volatiles
including sulfur oxides and water vapor from the spent
adsorbent. ~he temperature of the regeneration zone at the gas
- 10 inlet is maintained within the range of 800 - 1000F by
externally heating and recycling the desorbed volatiles through
the regeneration zone. A water vapor concentration of at least
40 mole percent and an SO2 concentration of at least 15 mole %
are maintained in the regeneration gas loop and the spent
adsorbent is continuously regenerated. A portion of the gas is
continuously removed from the regenerating gas loop at the rate
at which volatiles are desorbed to maintain ready-state
operation, and the gas removed ~rom the regenerating gas loop is
processed to recover the sulfur value contained therein. In a
preferred embodiment, the gas bleed stream, containing at least
40~ steam and at least 15% SO2, is passed into a reaction zone
wherein it is contacted with raw coal at a temperature within
the range of from 1150F to 1550F to convert the sulfur dioxide
to gaseous elemental sulfur and to partially oxidize the coal
.
-- 6 --
2~
wnich may then be used as adsorbent.
BRIEF DESCRIPTION OF THE D~AWINGS
The above description, as well as further objects,
features and advantages of the present invention, will be more
fully appreciated by reference to the following description of a
presently-preferred embodiment illustrative of the present
invention, when taken in connection with the accompanying
drawings, wherein:
Fig. 1 is a schematic diagram showing the flow of
materials in a pollution control system of the prior art for the
removal of sulfur oxides in flue gases; and
Fig. 2 is a schematic diagram showing the flow of
materials in a preferred embodiment of the regeneration system
invention.
~5 DESCRIPTION OF THE PREFERRED EMBODIMENTS
The carbonaceous adsorbent used in the present
lnvention is a partially oxidized coal. Exemplary of such
adsorbents is vanadium-treated anthracite derived RECOAL
(E'oster Wheeler trademark for by-product coal from its
'O RESOXR sulfur dioxide reduction process) which is preferred
from the viewpoint of its low cost. Another advantage of
RECOALTM is in its relatively high resistance to attrition
and the fines which it does generate are relatively
coarse and easily separated. This RECOALTM may have 0.04 to
about 0.25 weight % ~ deposited on it and ~ay be prepared
by its immersion in a sulfuric acid solution of V205 and
"drying" at about 1400F. An adsorbent preferred from the
viewpoint of its superior adsorption capacity is a char
commercially available from Bergbau Forschung (BF). The
BF char is derived from bituminous coal which is crushed
and then partially oxidized.
The regeneration zone is operated with a maximum
temperature, i.e., at the gas inlet, within the range of
800-1000F and, preferably, within a range of 800-850F.
The regenerating gas loop is initially charged
with an inert gas such as carbon dioxide, nitrogen or a mix-
ture thereof. If desired, steam may be added to the initial
charge. As the operation progresses, the initially charged
gas will be gradually displaced by recirculated desorbed
volatiles until a steady-state operation is achieved wherein
the composition in the regenerating gas loop will be approxi-
ately that of the desorbed gas.
By"steady-state operation" is meant that pressures
at gi~en points within the regenerator remain approximately
constant. To achieve steady-state operation, gas is removed
from the regenerator at a ~olumetric rate equal to that at
which the volatiles are desorbed from the spent adsorbent
in the regeneration zone. This is conveniently accomplished
by a pressure-responsi~e control system with the regenerating
gas loop maintained at approximately atmospheric pressure.
Typically, the pressure at the suction of the regenerating
gas blower (shown as 32 in Fig. 2) will be about 10" water
(gauge) and about 75"-100" water (gauge) at the blower
discharge.
-8-
In the des~ulfurization process of the present
invention flue gas is routed through a dust collector and
then through an adsorber wherein it is passed through a bed
of carbonac~ous adsorbent. The adsorbex may be of any
conventional type and may contain the carbonaceous adsorbent
as a fixed bed, moving bed or fluidized bed.
The spent adsorbent from the adsorber is routed
to the regeneration system of the present invention, a
preferred embodiment of which is deplcted in Fig~ 2 of the
drawings. The regeneration system includes a regenerating
gas loop 30 which recycles, by means of blower 32, the
main portion of the desorbed volatiles through the regenerator
34 and heat exchanger 36 which heats the gas entering the
regenerator to 800-1000F, preferably to 800-~50~F. The gas
in the regenexating gas loop, after it reaches steady-state
will have the compo~ition of the desorbed gas. This desorbed
gas typically contains 20-65 mole~ SO2 (dry basis), 50-75 mole
C2 (dry basis) and 50 mo~ ~ or more H2O (steam). A por-
tion of this gas is split off and routed to a RESOXR reactor
38 wherein it is reacted with crushed coal at 1150F to 1550Fto
- yield gaseous elemental sulfur which is then passed to a
condenser ~0 wherein it becomes liquified. Subsequently,
the liquid sulfur may be solidified by further cooling.
The RESOX reactor 40 and regenerator 34 are
preferably moving bed vessels, but they may also provide
means for fluidized bed or static bed operation. The
terminology "non-static bed", as used herein is meant to
cover both moving bed operation and fluidized bed operation.
For a definition of what i.s meant by moving bed operation
refer to column 5, lines 36-41,of V. S. 2,383,333 issued
April 21, 1959 to R.C. Oliver.
A more detailed description of the RESOXR process
pe~ se is provided in U. S . patent No. 4,147,762.
In the embodiment of Fig. 2 both the elemental
sul~ur and the RECOALT~ adsorbent are valuable by-products.
The RECO~TM is fo~med fr~m raw crushed coal in the RESOXR
reactor wherein it is partially oxidized and imparted a
surface porosity by ~he action of the SO2, 2~ and steam in the
bleed gas stream. The mole ratio of steam to SO2 is
preferably ln the range of 2:1 to 3:1 and the contact time
`~ is preferably 5.1 to 9.1 seconds. The operating parameters
are controlled to produce a RECOALTM product containing
50-60~ by weight of the oriainal coal.
. EX~LE 1
Anthracit~ derived RECOALTM was soaked in a
solution of 300 grams ~25~ 2000 cc sulfuric acid and 1000 cc
water. The wet RECOALTM was "dried" at 1400~ in an electric
furnace. In this manner three batches of RECOALTM were
prepared containing 0.11, 0.21 and 0.14 wt.4 V, respec~ively.
~ 20 ~a~ch No. 1 was prepa~ed from a RECOALTM which had been
: subjected to 50 cycles of SO2 adsorption and steam regenera-
; tion at 1300~F. Batch No. 2 was prepared from a RECOALTM
that had been subjected to 61 cycles and batch No. 3 was
pr~pared with fresh ~ECOALT~.
During the adsorption portion of each te~t cycle,
a simulated flue gas mixture composed of 0.2~ SO2, 15.0% CO2,
3.0% O , 10.0% H2O, and a balance of N2 (percentages being
--10--
v~ .
volume percentages) was fed at 275DF into a 2 cubic
foot vessel filled with RECOAL adsorhent as a fixed bed.
The gas residencetime (open volume) was 6 seconds.
Since two reactor vol~mes had been discharged duriny
regeneration, two batchwise adsorption tests per cycle
were required.
For regeneration, the reactor and its feed hopper
were filled to capacity. This required three reactor
volumes or batches of vanadium treated RECOALTM. The start-
up bed, which was held stationary within the reactor, wasbrought to temperature within a circulatillg mixture of hot
gases, composed of an initial charge of carbon dioxide
and evolving off~gas. As the temperature increased, more
gas was driven off the bed so that the composition of the
gas within the loop eventually approached that of the off-gas.
The recycle loop was maintained at constant pressure via its
pressure control valve
~ hen the solids within the reactor vessel reached
the proper temperature and test conditions were stabilized,
the vibrating feeder was turned on and the withdrawal of the
initial batch began. The second batch of the RECO~lTM which
was stored on the bottom of the hopper, started to flow by
gravity into the reactor ~essel. The testing began when the
second batch completely replaced the first batch and ended
when the third batch completely displaced the second batch in
the reactor vessel.
The third batch, which was now in the reactor,
became the start-up batch for the regeneration test of the
next cycle.
The ti~e interval spent wlthin the reactor vessel
varied from 1.5 to 2.75 hours (dwell time) for the test
batches of RECOALTM.
The results are shown in Table 1.
EXAr~LE 2
The same 2 cubic foot vessel was again used as
a boxed fixed bed adsorber and as a moving bed regenerator.
In this test BF char was used as the adsorber. For adsorp-
tion, the recycle blower was isolated from the other components
and a simulated flue gas mixture, identical to that employed
in Example 1, was fed at 250C through the fixed bed adsorber
filled with the BF char. The gas residence time (open volume)
was again 6 seconds.
For regeneration, the synthesi~ing flue gas system
~i 15 was isolated from the other components and the loop was placed
on line by circulating hot gases t:hrough the recycle loop.
The blower and the fired heater, both part of the loop,
provided the motive force and the necessary heat. The
circulating gas was at first carbon dioxide but changed in
composition to that of the regenerator off-gas as the bed
approached steady-state operation with a gas inlet tempera-
ture at 850F. Various batches of spent BF char from the
adsorber, containing from 4.4 to 5.56% by weight sulfur di-
oxide were fed at feed rates as high as 60 pounds per hour,
equivalent to a dwell time within the vessel of 1.3 hours.
The recycle blower which, along with the solids feeder,
determined the overall capacity of the power plant,
circulated up to 10 scfm of gas while developing a discharge
head of 75" - 100" water (gauge).
-12-
During the adsorption portion of the test, the
analysis of gas samples for sulfur dioxide content was
accomplished with two on-stream Theta sensors, Model U.S.
5,000. The results of seven test runs are shown in Table 2
below.
-13-
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